Galvanomagnetic devices are used as detector components for measuring electrical currents. Detailed information on such devices can be found in the technical literature, and reference may be made, for example, to Siemens' data book entitled "Galvanomagnetic devices" published 1976/1977. There are two different techniques for performing the measurement.
The first technique is one in which, at least in theory, there ought to be no need to perform compensation for variations in magnetic flux. The corresponding apparatus for implementing the technique essentially comprises a magnetic circuit including a gap in which the galvanomagnetic device is inserted. The magnetic circuit is excited by current flowing in a winding. Variations in the flux correspond to variations in the input current in accordance with the following equation: ##EQU1## where: B* is the value of the magnetic induction;
i is the input current at frequency f; PA1 .omega.=2.pi.f, i.e. the angular frequency; PA1 N is the number of turns in the winding; PA1 A.sub.s is the area of the magnetic circuit surrounded by said winding; and PA1 L is the inductance, including the effect of the magnetic circuit.
Given that the electric connection cables associated with the galvanomagnetic device constitute a loop whose area cannot be reduced completely to zero, and that there therefore remains a small residual loop area having a finite value A which is other than zero, the electrical voltage v.sub.i * induced between the output terminals is defined by the following expression which is applicable on the assumption that magnetic leakage (and thus leakage inductance) and also the increase in the effective gap area are negligible: ##EQU2##
Using the same assumptions, it is possible to express the voltage v.sub.i * as a fraction of the voltage v.sub.s induced in the said winding, i.e. ##EQU3##
Further, if the inductance L is conventionally expressed as the product of the square of the number of turns N.sup.2 multiplied by a constant K, i.e. L=KN.sup.2, it is also possible to express v.sub.i * that appears in equation (2) as follows, making use of equation (1): ##EQU4##
Equation (4) shows that the induced electrical voltage is directly proportional to the number of turns N and to the angular frequency .omega..
This means that the value of v.sub.i * is such that, in practice, it is generally necessary to provide compensation using techniques known to the person skilled in the art but contributing to further complication of the design of the measuring circuit.
Apparatuses of this category therefore suffer from the drawback of requiring a very high value of inductance (i.e. a high number of turns) in order to obtain the desired sensitivity for the measuring device: this condition gives rise to a relatively high value for the voltage induced between the terminals of the device and corresponds to behavior which is far from ideal.
The second technique of measuring electrical current using galvanometric devices is a technique in which variations of magnetic flux are compensated actively.
The corresponding apparatus for implementing the second technique differs from that used for implementing the above-described first technique in that the magnetic circuit includes an auxiliary winding in addition to the main excitation winding. A negative feedback circuit controls the current flowing through said auxiliary winding in such a manner as to provide full compensation for variations in magnetic flux, i.e. so as to obtain a value for the magnetic induction B which is zero or constant.
The negative feedback circuit must be designed in such a manner as to have a passband which goes from zero frequency (corresponding to D.C.) up to the desired frequency (for A.C.).
In any event, flux variation is very small (and in theory zero) such that the induced voltage is effectively compensated in practice.
Further, the impedance of the measuring device is also very low (and in theory zero).
However, although an apparatus for implementing the second technique behaves in a manner which is very close to ideal, practical realization of the above-mentioned negative feedback circuit is relatively complex because of the wide bandwidth that needs to be provided. Further, if high current values are involved, it is necessary for the auxiliary compensation winding to include a large number of turns in order to reduce the required compensation current, i.e. to reduce the power required for compensation purposes.
The range of applications for which said second category of apparatuses can be used is therefore limited by the above-mentioned constraints.
The object of the present invention is to provide a magnetically coupled current measuring apparatus which satisfies practical requirements better than known prior apparatuses for the same purposes, and in particular which:
requires no compensation for magnetic flux variation, and in particular which does not require the very complex compensating negative feedback circuits of the prior art to be used;
has performance close to that of an ideal current measuring apparatus (characterized by zero impedance) by having extremely low impedance, thereby eliminating the drawback which limited the use of prior art magnetically-coupled current measuring apparatuses due to their high impedance values; and
greatly reduced iron losses, thereby eliminating the drawback constituted by the limited range of materials that can be selected for the magnetic circuit.
Apparatus in accordance with the present invention uses a third measurement technique which may be referred to as a "passive" (or intrinsic) technique for compensating variations in magnetic flux, and which makes it possible to realize apparatuses (i.e. apparatuses belonging to a third category) whose design and structure are greatly simplified while still maintaining the advantages of apparatuses of the second category.
In addition, it should be emphasized that the negative feedback which they make use of intrinsically (i.e. the negative feedback due to their specific structure), is much more effective, particularly at high frequency, than the feedback which can be obtained from any kind of active negative feedback circuit.